1. Field of the Invention
This invention relates to an auto focus adjusting device for use in a camera or the like.
2. Background
Many of the auto focus adjusting systems of single-lens reflex cameras intend to focus the lens on an object to be photographed by repetitively effecting the cycles of "focus detection (sensor signal input and focus detection calculation) and lens driving". The amount of lens driving in each cycle is based on the defocus amount at a point of time whereat focus detection is effected in that cycle, and this presumes that the defocus amount during focus detection is eliminated when the lens driving is terminated.
As a matter of course, a considerable time is required for focus detection and lens driving, but in the case of a stationary object to be photographed, the defocus amount does not vary as long as the lens is not driven and therefore, the defocus amount to be eliminated at the point of time whereat lens driving is terminated is equal to the defocus amount at the point of time whereat the focus is detected and thus, correct focus adjustment is accomplished.
However, in the case of an object to be photographed which is rapidly moving, the defocus amount varies during focus detection and lens driving and said defocus amount to be eliminated sometimes differs remarkably from the detected defocus amount and as a result, there arises the problem that the lens is not focused on the object to be photographed when lens driving is terminated.
Auto focus adjusting methods which intend to solve the above-noted problem are disclosed in Japanese Laid-Open Patent Applications Nos. 62-125311, 62-139512 and 62-139511.
The gist of the methods disclosed in the abovementioned patent applications is that in view of the variation in the detected defocus in each said cycle and the time interval between said cycles, the variation in defocus attributable to movement of the object to be photographed is foreseen and correction is exerted on the amount of lens driving, and from the viewpoint of the accuracy of the focus at the end of lens driving, the above-noted problem is expected to be solved by the same methods.
However, if zooming is effected when the auto focus adjusting operation by the above-described correcting method is performed with a zoom lens mounted as a photo-taking lens, there will arise the following problem.
In the above-described correcting method, the variation in defocus attributable to movement of the object to be photographed is foreseen from a plurality of defocus amounts detected in the past, but if zooming is effected, the focal length changes and therefore, the variation in defocus changes even for the same movement of the object to be photographed, and proper correction cannot be accomplished.
FIG. 1 of the accompanying drawings is a graph for illustrating the method of correcting the amount of lens driving shown in the assignee's prior U.S. application filed on Oct. 19, 1988. In this graph, the abscissa represents time t and the ordinate represents the imaging plane position d of the object to be photographed.
The locus f(t) indicated by the solid line means the imaging plane position of the object to be photographed, and the locus l(t) indicated by the broken line means the imaging plane position of the lens.
More particularly, f(t) means the imaging plane position, at time t, of an object to be photographed which approaches the camera in the direction of the optic axis when the focus adjusting optical system of the photo-taking lens is at a position for forming the focus at infinity, and l(t) means the imaging plane position of the same object to be photographed at the position of the focus adjusting optical system at time t. The section [t.sub.1, t.sub.1 '] corresponds to the focus detecting operation, and the section [t.sub.i ', t.sub.i+1 ] corresponds to the lens driving operation.
Accordingly, the difference in the direction of the vertical axis d between f(t) and l(t) at the same time t is the so-called defocus amount. DF.sub.i represents the detected defocus amount at time t.sub.i, DL.sub.i represents the amount of lens driving as converted into the imaging plane position which has been executed from the result of the focus detection at time t.sub.i-1, and TM.sub.i represents the time interval between the focus detecting operations.
In the example shown in FIG. 1, it is assumed as a premise for effecting the correcting calculation that the imaging plane position of the object to be photographed varies in accordance with a quadratic function. That is, it is assumed that if the current and past three imaging plane positions (t.sub.1, f.sub.1), (T.sub.2, f.sub.2) and (t.sub.3, f.sub.3) are known at time t.sub.3, the imaging plane position f.sub.4 at time t.sub.4 can be foreseen.
However, what the camera can detect in reality is not the imaging plane positions f.sub.1, F.sub.2 and f.sub.3, but the defocus amounts DF.sub.1, DF.sub.2 and DF.sub.3 and the amounts of lens driving DL.sub.1 and DL.sub.2 as converted into the amounts of movement of the imaging plane. Time t.sub.4 is strictly a future value, and actually is a value which varies with a variation in the accumulation time of an accumulating type sensor caused by the brightness of the object to be photographed, but when determining f.sub.4, it is assumed as known in the relation that t.sub.4 -t.sub.3 =t.sub.3 -t.sub.2, for simplicity.
Under the above-described assumption, the lens driving as converted into the amount of movement of the imaging plane when lens driving is effected toward t.sub.4 at time t.sub.3 ' from the result of the focus detection at time t.sub.3 is found in the following manner. EQU a.multidot.t.sup.2 +b.multidot.t+c=f(t) (1) EQU a.multidot.t.sup.2.sub.1 +b.multidot.t.sub.1 +c=f(t.sub.1) (2) EQU a.multidot.t.sup.2.sub.2 +b.multidot.t.sub.2 +c=f(t.sub.2) (2') EQU a.multidot.t.sup.2.sub.3 +b.multidot.t.sub.3 +c=f(t.sub.3) (2")
When, in FIG. 1, the point l.sub.1 is considered to be the origin, EQU f.sub.1 =DF.sub.1 (3) EQU f.sub.2 =DF.sub.2 +DL.sub.1 (3') EQU f.sub.3 =DF.sub.3 +DL.sub.2 +DL.sub.1 (3") EQU t.sub.1 =O (4) EQU t.sub.2 =TM, (4') EQU t.sub.3 =TM.sub.1 +TM.sub.2 (4")
If equations (3), (3'), (3"), (4), (4') and (4") are substituted into equations (2), (2') and (2") to find said a, b and c, ##EQU1## EQU c=DF.sub.1 (7)
Consequently, the amount of lens driving DL.sub.3 as converted into the amount of movement of the imaging plane at time t.sub.4 is: ##EQU2##
Here, on the assumption that as previously described, TM.sub.3 is known in the relation that TM.sub.3 =TM.sub.2, DL.sub.3 is found from equation (8). The amount of lens driving at t.sub.n after time t.sub.4 can likewise be found from the past three detected defocus amounts DF.sub.n-2, DF.sub.n` and DF.sub.n, the past two actual amounts of lens driving DL.sub.n-2 and DL.sub.n-1 and the past two time intervals TM.sub.n-2 and TM.sub.n-1. ##EQU3##
If in accordance with equations (9), (10) and (11), the defocus amount DL.sub.n for effecting lens driving is found from the detected defocus amount DF.sub.n and lens driving is effected, proper focusing will always become possible at the end of lens driving.
In the above-described correcting method, the imaging plane position is extrapolated by a quadratic function and therefore, the data of the past two focus detecting operations are necessary However, data is deficient at the first two times after focus adjustment is started and therefore, as shown in FIG. 1, in the first two focus adjusting operations, the lens is driven on the basis of the detected defocus amount itself Accordingly, the actual correcting calculation is effected from the third lens driving on, and as expressed in FIG. 1, the effect of correction presents itself from time t.sub.4 on.
Now, the change in the imaging plane position shown in FIG. 1 is that in the case of a particular focal length, and even for the same movement of the object to be photographed, the aspect of the change in the imaging plane position will change if the focal length of the photo-taking lens changes. Assuming that FIG. 1 is the case of the relatively telephoto side of a zoom lens, the movement of the imaging plane position on the wide angle side is as shown in FIG. 2 of the accompanying drawings. Even for the same movement of the object to be photographed, the variation in defocus becomes smaller on the wide angle side than on the telephoto side.
The actual locus f(t) in FIG. 3 of the accompanying drawings shows the change in the imaging plane position when the zoom position moves from the telephoto side to the wide angle side at time t.sub.x1 to time t.sub.x2. That is, before time t.sub.xl, the imaging plane position was as shown in FIG. 1, but from time t.sub.x2 on, it shifts to the imaging plane position shown in FIG. 2. At the same time, the imaging plane position l(t) of the lens also changes from time t.sub.x1 to time t.sub.x2. Therefore, the lens driving DL.sub.2 based on the detected defocus amount DF.sub.2 at time t.sub.2 is as shown in FIG. 3 and the imaging plane position of the lens is l3.
That is, the detected defocus amount DF.sub.2 at time t.sub.2 is the defocus amount on the telephoto side and thereafter, the zoom position is set to the wide angle side and therefore, the amount of lens driving in this case is DL.sub.2 differing from DL.sub.2 ' corresponding to said defocus amount DF.sub.2 detected on the telephoto side, and the imaging plane position of the line is l3.
On the other hand, as regards the imaging plane position of the lens found by calculation, the imaging plane position of the lens at time t.sub.2 is l.sub.2, and the defocus amount at time t.sub.2 is DF.sub.2 and the amount of lens driving corresponding to the then DF.sub.2 (telephoto side) is DL.sub.2 ' and therefore, the calculation of l.sub.2 +DL.sub.2 '+l.sub.3 ' is effected.
Also, assuming that the defocus amount detected at time t.sub.3 is DF.sub.3, the imaging plane position at time t.sub.3 is f.sub.3 =l.sub.3 +DF.sub.3, but the imaging plane position based on the imaging plane position l.sub.3 ' of the lens found by said calculation is f.sub.3 '=l.sub.3'+DF.sub.3 ' (DF.sub.3 =DF.sub.3 '). Consequently, the imaging plane position f.sub.4 ' at time t.sub.4 found from these calculated data by the correcting calculation differs from the imaging plane position f.sub.4 at the actual time t.sub.4, and this results in the occurrence of an inconvenience.